Both radioimmunotherapy and the unlabeled monoclonal antibody, rituximab, have clinical activity in low-grade lymphomas. Theoretically, patients who have been previously treated with rituximab may mount an immune response that would preclude subsequent uses of other antibody treatments. This study is being performed to determine the activity and safety of Iodine-131 Anti-B1 Antibody therapy in patients who have been previously treated with the anti-CD20 monoclonal antibody, rituximab.